Diazonium salt compounds have extremely high chemical activity, and therefore react with compounds such as phenol derivatives and compounds having an active methylene group (generally called couplers) to readily form azo dyes. Since diazonium salt compounds also are light-sensitive and lose their activity by irradiation with light, they are widely utilized as photo-recording materials as described, for example, in Nippon Shashin Gakkai, "Shashin Kogaku No Kiso--Hi-gin-en Shashin Hen--," Corona Sha (1982) pp.89-117, pp.182-201. Due to these properties, diazonium salt compounds have also recently been applied to thermal recording materials requiring image fixation, and a thermal recording material has been proposed in which a diazonium salt compound is thermally reacted with a coupler to form an image, which is then fixed by irradiation with light (e.g., Koji Sato et at., Gazo Denshi Gakkai-shi, Vol.11, No.4 (1982) pp.290-296). However, these recording materials employing diazonium salt compounds are disadvantageous in that the diazonium salt compounds gradually undergo pyrolysis to lose their reactivity even in the dark due to their extremely high activity. That is, such recording materials have a short shelf life.
As expedients for stably incorporating a diazonium salt into a recording material, various techniques have been proposed. Among the most effective is a technique of microencapsulating a diazonium salt compound. In this manner, the microencapsulated diazonium salt can be isolated from substances which accelerate pyrolysis, such as water or a base, resulting in a significantly improved shelf life (Tomomasa Usami et al., Denshi Shashin Gakkai-shi, Vol.26, No.2 (1987) pp.115-125).
In a process generally employed for microencapsulating a diazonium salt compound, the diazonium salt is dissolved in a hydrophobic solvent (oil phase) and the solution is added to an aqueous solution of a water-soluble polymer (aqueous phase). The mixture is then emulsified with a homogenizer or the like. This emulsification is conducted in the presence of a monomer or prepolymer which serves as a microcapsule wall material and has been added prior to mixing to one or both of the oil phase and the aqueous phase. Thus, a polymerization reaction or polymer precipitation occurs at the interface between the oil phase and the aqueous phase, to thereby form polymer walls and obtain microcapsules. Such methods are described in detail, e.g., in Asashi Kondo, "Microcapsule", Nikkan Kogyo Shinbun-sha (1970) and Tamotsu Kondo, "Microcapsule", Sankyo Shuppan (1977). For forming microcapsule walls, various materials can be used such as crosslinked gelatin, alginic acid salts, cellulose derivatives, urea resins, urethane resins, melamine resins and nylon resins. Microcapsules having capsule walls made of a resin having a glass transition temperature slightly higher than room temperature, e.g., a urea or urethane resin, are called heat-responsive wall microcapsules because the capsule walls are impermeable at room temperature but become permeable at a temperature not lower than the glass transition temperature to cause physical mixing of the diazonium compound with the coupler and basic compound arranged outside the microcapsules. This kind of microcapsule is useful in a thermal recording material. For example, heat-responsive wall microcapsules containing a diazonium salt are applied onto a substrate along with a coupler and a base to produce a recording material. As a result, the diazonium salt can be contained in the recording material in a stable manner over a prolonged time period and, in addition, easy color development by heating and image fixation by light irradiation become possible.
Although the stability of a diazonium salt compound can be greatly improved by microencapsulation as described above, some problems still remain. One problem is that the diazonium salt partly decomposes during the microencapsulation reaction. Another is that the shelf life is not always satisfactory when the recording material is stored either over a prolonged period of a few years or more or under unfavorable environmental conditions such as high-temperature and high-humidity conditions.